Loading additem.pl is slow when loading a record with 350+ items. With Plack Koha, it's taking about 8.5 seconds at the moment. I haven't really investigated yet, but it's not immediately obvious why it's slow. My guess is probably C4::Biblio::EmbedItemsInMarcBiblio, but I haven't delved into the depths.
While I've noticed this, I don't have a sponsor for this work, so I'm probably not going to work on it. Just noting it here as a problem.